1. Field of the Invention
The invention relates to a liquid crystal display using a compensating film that prevents light leakage at a viewing angle and at the polarizing plate and improves the color characteristics at the viewing angle.
2. Description of the Related Art
Liquid crystal (LC) molecules have an anisotropy based upon their asymmetric shape. The optical anisotropy of an LC cell containing a film of such LC molecules changes depending on the distribution and tilt angle of liquid crystal molecules.
The above properties constitute important factors toward changing the polarization of light according to the viewing angle of a cell or a film of LC. Due to the inherent properties of the LC, variations in the brightness and the contrast ratio according to upper, lower, left and right viewing angles are caused during the operation of a liquid crystal display (LCD). These variations in brightness and contrast acts as one of the most disadvantageous properties of the LCD.
FIG. 1 shows a graph illustrating a viewing angle characteristic of an LCD according to the related art. As shown in FIG. 1, when a black is attained in a front side at a viewing angle of 0°, light leakage generates at a predetermined viewing angle, even from the polarizing plates that are perpendicular to each other. In particular, the light leakage maximizes at a viewing angle of 70°.
To addresses this problem, a method where a compensating film to compensate for the anisotropic distribution was devised.
FIG. 2 shows a schematic view illustrating a structure of an LCD provided with a compensating film according to the related art. Referring to FIG. 2, the related art LCD includes: a liquid crystal display (LCD) panel having an upper substrate 22, a lower substrate 23 spaced apart a predetermined distance from the upper substrate 22, and a liquid crystal layer 21 interposed between the upper substrate 22 and the lower substrate 23. Compensating films 24a and 24b are respectively attached on both outer surfaces of the LCD panel, and first and second polarizing plates 25 and 26 are attached on the compensating films such that an optical transmission axis of the first polarizing plate 25 is perpendicular to an optical transmission axis of the second polarizing plate 26.
The compensating films 24a and 24b have an anisotropic distribution opposite to that of the LC cell, if possible, and are made to remove a retardation difference of light according to the viewing angle when they are coupled with an LC cell.
In general, a compensating film made of polymer influences a variation in the phase difference with respect to the transmission light, and the compensating film extends in a predetermined direction to have birefringence due to an anisotropic orientation of molecules.
For example, when an external electric field is applied to a normal black mode twisted nematic (TN) LCD, liquid crystal molecules are aligned in response to the electric field, so that a light transmission is generated by the equations below:I=I0 sin2[θ(1+u2½],
      u    =                  π        ⁢                                  ⁢        R                    θ        ⁢                                  ⁢        λ              ,R=Δn·d 
where I is the intensity of a transmission light, I0 is the intensity of an incident light, Δn is the birefringence, d is the thickness of an LC cell, λ is the wavelength of the transmitted light, θ is the twist angle of a twisted nematic LC, and R is the phase difference.
The above equations show that since the phase difference has a close relationship with the viewing angle, a compensation of the phase difference is desirable for an improvement of the viewing angle.
The compensating films disposed between the LCD panel and the polarizing plate, for the purpose of the compensation of the phase difference, uses a uniaxial birefringent anisotropic material and a biaxial birefringent anisotropic material.
FIGS. 3A through 3C show a refractive anisotropic ellipsoid of a phase difference compensating film. As shown in FIGS. 3A through 3C, the refractive indexes in x, y, z-direction of a Cartesian coordinate are nx, ny and nz, the uniaxial property and the biaxial property are determined by whether or not nx is identical to ny. In other words, as shown in FIG. 3A, if refractive indexes nx and ny of the x and y-directions are identical to each other and their size is different from the size of the refractive index nz of the remaining direction z, it is called ‘uniaxial.’ As shown in FIGS. 3B and 3C, when the refractive indexes nx, ny and nz in the three directions are different from one another, it is called ‘biaxial.’
The generally used compensating film using a uniaxial refractive index anisotropic material has the long axis of an ellipsoid, which is parallel to or is perpendicular to a surface of the film.
In the related art method of manufacturing a compensating film, a polymer film extends uniaxially or biaxially thereby obtaining the desired refractive index such that the optical axis of the phase difference film has an arbitrary angle with respect to the advancing direction of the film.
Phase difference films made as described above have optical axes that are parallel to or are perpendicular to the extending direction of the film. Hence, to use the phase difference films for the purpose of optical compensation in an LCD, the optical axes of the phase difference film must form an arbitrary angle with the optical axes of the polarizing plate. It is accordingly necessary to specially cut the manufactured phase difference films.
However, since the above method adjusts the extension ratio mechanically, it is difficult to control the orientation angle to achieve a desired angle. Also, the phase difference films are not attached from a roll, but one sheet at a time must be attached to the polarizing plate, and disadvantages in process efficiency and the management of foreign particles result.